1. Field of the Invention
The present invention relates to a bias circuit that compensation for change in threshold voltage of a transistor and temperature compensation can be accomplished.
2. Description of the Related Art
When an applied voltage to a transistor is fixed in an integrated circuit (IC), there are two problems. That is, one is the change in characteristics of the integrated circuit and decrease in a production yield due to deviation in threshold voltage (Vth) of the transistor, and the other is change in characteristics of the integrated circuit due to use temperature change. Therefore, it is required to compensate for both of the threshold voltage change of the transistor and the temperature change.
FIG. 1 is a circuit diagram showing a conventional bias circuit with a temperature compensation function (IEEE TRANS. MTT, VOL.49, No12, December 2001). The bias circuit shown in FIG. 1 is connected with a gate bias point 106 of an RF amplifying transistor 104 in an amplifier 151, and has high impedance enough for a high frequency signal due to the resistance 108. The bias circuit includes a diode 202 and a resistance 204. A voltage 201 is applied to the anode of the diode 202, and a voltage 205 is applied to the cathode of the diode 202 through the resistance 204. A node 203 between the cathode and the resistance 204 is connected to the gate bias point 106 of the RF amplifying transistor 104 through the resistance 108.
The amplifier 151 includes a capacitor 107, the RF amplifying transistor 104, and the resistance 108. A radio frequency signal is inputted to one end of the capacitor 107. The gate bias point 106 of the RF amplifying transistor 104 is connected with the other end of the capacitor 107 and one end of the resistance 108. The drain thereof is connected to one end of a resistance 102. A voltage 101 is applied to the drain of the RF amplifying transistor 104 through the resistance 102, and a voltage 105 to the source thereof.
The bias circuit shown in FIG. 1 utilizes a characteristic of the diode shown in FIG. 2, in which a forward voltage Vf increases when temperature decreases in a state that a forward current is kept constant. Generally, a current-voltage characteristic of a Scotty barrier diode is expressed byIf=Js(exp(qVf/kT)−1)where Js=AT2exp(−qΦB/kT), If is a forward current, q is unit electric charge, Vf is a forward voltage, k is the Boltzmann's constant, T is temperature, A is the effective Richardoson's constant, φB is a Scotty barrier height of the diode. That is, the voltage drop in the diode 202 becomes large when the temperature decreases, and therefore the voltage at the node 203 on the side of the cathode of the diode 202 lowers. As a result, the voltage V106 of the gate bias point 106 lowers, so that the gain of the transistor 104 is restricted. On the other hand, the voltage drop at the diode 202 becomes small when the temperature increases. Therefore the gate bias voltage V106 rises so that the gain of the transistor 104 becomes large.
FIG. 3 shows a dependency on the temperature change, of drain current 103 in the transistor 104 (Id: unit mA) using the bias circuit shown in FIG. 1. As shown in FIG. 3, the drain current 103 is restricted with the temperature decrease. In the other words, the bias circuit shown in FIG. 1 has the temperature compensation effect.
Next, FIG. 4 is a circuit diagram showing a conventional bias circuit with a compensation function of a change in threshold voltage (2003 IEEE MTT-S Digest TU5B-3, pp. 121–124). The bias circuit shown in FIG. 4 is connected with the gate bias point 106 of the RF amplifying transistor 104 in the amplifier 151 and has high impedance enough for a high frequency signal by the resistance 108. The bias circuit includes a transistor 305 having a same DC characteristic as the RF amplifying transistor 104, a resistance 303, and a resistance 306. A voltage 301 is applied to the drain of a transistor 305 through the resistance 303. A voltage 308 is applied to the source of the transistor 305. A voltage 307 is applied to the gate of the transistor 305 through the resistance 306. It should be noted that the amplifier 151 in FIG. 4 is the same as shown in FIG. 1.
In the bias circuit shown in FIG. 4, when the threshold voltage Vth of the transistor changes by ΔVth, the circuit is set to satisfy the relational of ΔId2*R303 =ΔVth where ΔId2 is a difference of the drain current 302, and R303 is the resistance value of the resistance 303. When the drain current 302 becomes large by ΔId2 with the decrease of the threshold voltage Vth, the voltage 304 lowers by ΔVth. As a result, the voltage at a gate bias point 106 of the transistor 104 becomes low by ΔVth. On the other hand, when the threshold voltage Vth increases, the threshold voltage change ΔVth is compensated oppositely.
FIG. 7 shows a dependency upon the threshold voltage change (ΔVth: unit V), of the drain current 103 of the transistor 104 (Id: unit mA) when the bias circuit shown in FIG. 4 is used.
FIG. 5 is a circuit diagram showing a conventional circuit (of a self-bias method) which has both of a threshold voltage change compensation function and a temperature change compensation function (by Yasuyuki Itou, et al., “Base and application of MMIC technology”, May 31, 1996 Realize company, P. 130). In the circuit shown in FIG. 5, a resistance 406 is connected in series between the source of a RF amplifying transistor 404 and a ground potential GND. A capacitor 407 is connected to the ground in parallel to the resistance 406 for a high frequency signal. Also, a voltage 401 is applied to the drain of the RF amplifying transistor 404 through the resistance 402. A radio frequency signal is supplied to the one end of a capacitor 408. The other end of the capacitor 408 is connected to the gate of the RF amplifying transistor 404.
In the circuit shown in FIG. 5, the following functions are accomplished when the drain current 403 of the transistor 404 is changed due to a temperature change and a threshold voltage change. For instance, when the drain current 403 increases, the voltage 405 becomes high, so that a voltage difference Vgs between the gate and the source in the transistor 404 decreases. As a result, the drain current 403 decreases. On contrary, when the drain current 403 decreases, the voltage difference Vgs becomes large so that the drain current 403 increases. That is to say, the compensation functions in the circuit shown in FIG. 5 are accomplished to keep the drain current 403 of the transistor 404 constant.
FIG. 6 is a circuit diagram showing another conventional circuit which has functions compensating both of threshold voltage change and temperature change (2002 IEEE MTT-S Digest TH1B-4, pp. 1427–1430). The circuit shown in FIG. 6 is a bias circuit which is connected with a gate bias point 106 of an RF amplifying transistor 104 in an amplifier 151. The bias circuit includes a first circuit which includes a transistor 504 having a same DC characteristic as the RF amplifying transistor 104, a resistance 502 connected with the drain of the transistor 504, a diode 506, and a resistance 509 connected with the source of the transistor 504, and a second circuit which includes a resistance 511 and a diode 513, which are connected with the gate of the transistor 504.
The gate bias point 106 of the transistor 104 is connected to a node 503 between the drain of the transistor 504 and the resistance 502 through the resistance 108 to have high impedance enough for a high frequency signal. The drain of the transistor 504 is grounded through the resistance 502. The anode of the diode 506 in the first circuit is grounded and the cathode of the diode 506 is connected with a negative voltage 514 through the resistance 509. The source of the transistor 504 is connected to the node 507 between the diode 506 and the resistance 509 in the first circuit. The cathode of the diode 513 is connected to the negative voltage 514, and the anode of the diode 513 is grounded through the resistance 511 in the second circuit. A gate of the transistor 504 is connected to a node 512 between the resistance 511 and the diode 513 in the second circuit. The amplifier 151 in FIG. 6 is the same as shown in FIGS. 1 and 4.
When the threshold voltage Vth of the transistor is changed by ΔVth in the bias circuit shown in FIG. 6, the threshold voltage change ΔVth is compensated for to satisfy the relational expression of ΔId5*R502=ΔVth, where ΔId5 indicates a change of the drain current 501, and R502 indicates the resistance value of the resistance 502. The temperature characteristic of a forward voltage Vf of the diode (shown in FIG. 2) is used when the temperature change has occurred. For instance, the voltage drop across the diode 513 becomes large when the temperature decreases. Therefore, the voltage at the node 512 corresponding to a gate voltage of the transistor 504 increases. The voltage drop across the diode 506 becomes large similarly. Therefore, the voltage at the node 507 corresponding to the source voltage of the transistor 504 decreases. As a result, the voltage between the gate and the source in the transistor 504 increases, so that the drain current 501 of the transistor 504 is increased, resulting in lowering the voltage at the node 503. This is because the voltage drop indicated as the product of the resistance 502 and the drain current 501 becomes large. Therefore, the voltage at the gate bias point 106 which has the same voltage as the voltage 503 decreases in the RF amplifying transistor 104. Consequently, the effect of the temperature compensation is achieved. Thus, the bias circuit shown in FIG. 6 has a relation of drain current and threshold voltage change shown in FIG. 8 and a relation of drain current and temperature change shown n FIG. 9.
Furthermore, Japanese Laid Open Patent Application (JP-P2001-168699A) discloses a technique to maintain a stable operation in spite of the temperature change of the threshold voltage Vth of the transistor in a MOSFET for power supply. In the conventional technique, resistances 19, 17, and 16, diodes 21 and 22, and a zenar diode 20 are connected with the gate of MOSFET1, and the threshold voltage change compensation can be achieved in the temperature change.
However, there are the following problems in the above-mentioned conventional techniques. That is, generally, when the threshold voltage change ΔVth has occurred in the transistor, the change in the characteristics of the transistor occurs if the bias voltage is not changed by the threshold voltage change ΔVth of the transistor for the RF amplification. The circuit of FIG. 1 has the compensation effect to the temperature change but does not have the compensation effect to the threshold voltage change. Therefore, the drain current 103 decreases, which cause a change in characteristic as the threshold voltage Vth becomes shallow.
Generally, a temperature coefficient of an epitaxial resistance formed on a GaAs substrate has a positive coefficient. In the bias circuit of FIG. 4, When the rise of temperature is caused, the values of a drain current 302 of transistor 305 and resistance 303 become large and a product of the drain current 302 and the resistance 303 becomes large. That is, a voltage drop across resistance 303 becomes large. As a result, as the temperature increases, the gate bias 106 of the transistor 104 for the RF amplification becomes low to decrease the drain current 103, resulting in the degradation of the RF characteristic. In this way, in case of the bias circuit of FIG. 4, there is not a compensation effect to the temperature change.
Also, when the gain of the transistor 104 should be kept constant to the temperature change, it is needed to decrease the drain current 103 with the temperature decrease. In the bias circuit of FIG. 5, the temperature change compensation effect is insufficient because the bias circuit has only a function to keep the drain current 103 constant to the temperature change.
In the bias circuit of FIG. 6, three current paths for the drain current 501 of the transistor 504, for a current 505 flowing through the diode 506 and the resistance 509, and for a current 510 flowing through the resistance 511 and the diode 513 are needed. Therefore, the consumption current becomes large.
In the technique disclosed in Japanese Laid Open Patent Application (JP-P2001-168699A), it is possible to compensate for the threshold voltage change to the temperature change but it not possible to compensate for the threshold voltage change to change on the manufacturing.